Recovery of pure hydrocarbons by extractive distillation



Jan. 9, 1951 .1. GRISWOLD 2,537,459

RECOVERY OF PURE HYDROCARBONS BY EXTRACTIVE DISTILLATION Filed Feb. 5,1946 4. K ova/1540 /aooucr SOLVE/V7 RECYCLE r550 I n v 2 7 aorrousnooucr FIG. I

3a 40 35 as FIG- Ir. JOHN GR/SWOLD, JNVENTOR.

Patented Jan. 9, 1951 RECOVERY OF PURE HYDROCARBONS BY EXTRACTIVEDISTILLATION John Griswold, Austin, Tex. Application February 5, 1946,Serial No. 645,660

1 Claim.

This invention is directed to a method or process for separatingnarrow-boiling mixtures of those non-aromatic hydrocarbons which occurin straightrun and in processed petroleum fractions, in synthetichydrocarbon mixtures such as the products of hydrogenation or of theFischer- Iropsch synthesis, and in pyrolized or catalytically convertedpetroleum fractions.

Thus, the various types of hydrocarbons to' which the invention relatesare paraffins, naphthenes (cycloparafiins) unsaturated naphthenes,olefins, and diolefins, or, more generally, those non-aromatichydrocarbons whose boiling points are in the gasoline range, and whichhave about five to nine carbon atoms in the molecules.

The principal object of the invention is to recover from mixtures,however derived, of such hydrocarbons, each of the several hydrocarboncomponents in a substantially pure state unmixed with others of suchcomponents or with foreign substances.

A further object is the provision of a method for separating frommixtures of the above enumerated types of hydrocarbons the variouscomponents thereof by first separating the mixtures into portions orfractions, each containing but a single type of hydrocarbon, and thenfurther treating the portions or fractions individually to separate fromeach other their constituent hydrocarbons and recover them insubstantially pure state. It will be understood, however, thatreferences herein to hydrocarbons, or types of hydrocarbons, "insubstantially pure state denote materials whose purities are equivalentto technical grades or better, but at least 90 mol per cent; however,individual hydrocarbons recovered in accordance with the invention arefrequently of purity above 98 mol per cent.

It is well known that petroleum distillates boiling within the gasolinerange (say, 20 C. to 150 0.) usually are composed of an extremely largenumber of hydrocarbons of several distinct types. Even a narrow-boilingfraction (say, of 5 C. boiling range) which has been rigorouslyfractionated from such a distillate contains quite a number ofhydrocarbons, of which at least one pair and usually several pairs arepresent whose closeness of boiling points or whose partial pressurerelations are such that the component hydrocarbons cannot besatisfactorily separated and recovered in substantially pure state byconventional distillation processes.

In accordance with this invention however,

the component hydrocarbons may be recovered individually fromnarrow-boiling mixtures thereof by a succession of operations includingconventional fractional distillation and extractive distillation in thepresence of a relatively inert foreign liquid vehicle or solvent.

Distillation of a hydrocarbon mixture of the character mentioned, in thepresence of a substantial quantity of a higher boiling miscible vehicleor solvent is essential to the invention as hereinafter more fullydescribed, the object of this operation being to separate theclose-boiling hydrocarbons constituting the original mixture or feedinto a plurality of portions or fractions each containing not more thanone of said types of hydrocarbons.

It is known that the various types of nonaromatic hydrocarbons havedifferent activity coefficients or Raoults law deviation in a solvent.In most, but not in all cases, numerical values of activity coefficientsof said types of hydrocarbons lie in the same order as thehydrogen/carbon ratio of said types and of the individual hydrocarbonstherein. Those hydrocarbons which are high in hydrogen content, e. g.,parafiins, have relatively high activity coefiicients and are morevolatile from the solvent than are the other types, whereas hydrocarbonswhich are low in hydrogen content, c. e., aromatics, have relatively lowactivity coefficients and are less volatile from the solvent than arethe other types. Naphthenes, unsaturated naphthenes, olefins anddiolefins have activity coefllcients intermediate between those ofparafllns and of aromatics. Th activity coefficients of two isomers ofthe same type and molecular weight, e. g., 2-methylpentane and3-methylpentane are substantially the same, and when they occur togetherin substantial proportions in the same mixture their separation fromeach other thus may offer some difliculties, but their separation, as atype or group, from other constituents is readily eflectuated inaccordance with the invention.

However, provided sufliciently large numerical differences betweenactivity coefiicients of said types of hydrocarbons exist; said types inac-.

cordance with the invention can readily be separated from each otherwhere present in nar row-boiling fraction (say, of boiling range notgreater than 15C. or 20 C.). Forexample, the activity coefllcients ('y)of various types of hydrocarbons as defined by the relation,

P= vapor pressure of subject type of hydrocarbons fl= mol fraction ofsubject type of hydrocarbons in vapor :r=mol fraction of subject type ofhydrocarbons in liquid for atmospheric pressure, aniline as the solvent,and 80 mol per cent aniline in the mixture, are approximately:

In general, activity coemcients of all types increase with increasingsolvent concentration in the liquid, so that a high proportion ofsolvent (such as 80 mol per cent) favors hydrocarbon separation.

More specifically, the practice oi this invention comprises (1) initialseparation of a narrowboiling mixture into two or more portions in suchmanner that at least one of said portions contains substantially fewertypes of hydrocarbons than the original mixture, or but one type ofhydrocarbon, or, one hydrocarbon in substantially pure state; (2) theseparation of any of said portions or groups containing more than onetype into subgroups each in turn containing fewer hydrocarbon types thanthe parent portion, or but one substantially pure hydrocarbon; (3) therecovery of substantially pure individual hydrocarbons from each of thegroups containing but a single hydrocarbon type.

That part of the invention herein designated extractive distillation maybe accomplished with the aid of a unit comprising a fractionating columnof any of the well-known types, such as a bubble-tray column. Theessentials of a unit appropriate to the performance of the operation areshown diagrammatically in Fig. i of the accompanying drawing withauxiliary apparatus such as heat exchangers, pumps, tanks, valves andcontrol equipment omitted for simplicity.

In Fig. 2 of the drawing a more complete layout of apparatus adapted forperforming the invention is shown diagrammatically comprising,

with other apparatus, three interconnected units of the type representedin Fig. 1. The said apparatus, now to be more fully described, is welladapted but not necessarily specifically required for the practice ofthe invention, and the following descriptions of it and its operationsare therefore to be considered merely as illustrative of the nature ofthe invention and a preferred manner of carrying out the operations itcontemplates.

In accordance with the invention I employ, as indicated, a suitablesolvent for the hydrocarbon feed. The required properties of the solventare (1) that the hydrocarbons be soluble therein; (2) that the activitycoefiicients of the various types of hydrocarbons be essentiallydifferent from one another when dissolved therein, in other words, thatselectivity be one of its attributes; (3) that it be reasonably stableand not highly corrosive under conditions of use and recovery; (4) thatit be substantially inert toward all hydrocarbons present in theoriginal mixture in appreciable quantity; (5) that it be of suflicientlyhigher boiling point than any of the hydrocarbons in the mixture toinhibit formation of azeotropes with any of them. and (6) that itpreferably be easily separable and puriflable by distillation or otherprocess or by simple combinations of processes. Examples of suitablesolvents are aniline,

dichloroethyl ether, furfural, phenol and nitrobenzene, although manyother liquid organic compounds or mixtures of organic compoundssatisfying the above requirements are known in the art and may beemployed and the following description of the practice of the inventionis directed to operation with the first mentioned, namely aniline, as asolvent.

The apparatus illustrated in Fig. 1 comprises a column I which may beany suitable type of fractionating reflux distillation column, providedwith feed line 2, reboiler 3 or other suitable means for supplyingsupplemental heat, and an overhead vapor condenser 4 from which line 5carries part of the overhead condensate back to column I near the top ofthe column, while a line 6 from the condenser affords an outlet for theremainder of the condensate.

Between the points in the column at which the feed and condensate areadmitted through lines 2 and 5 respectively, a line I returns to thecolumn solvent withdrawn therefrom through line 8 during extractivedistillation.

Solvent withdrawn through line 8 carries with it the less volatileconstituents of the feed, with the exception of such relativelyinsignificant portions thereof as may escape with the overhead productwithdrawn through line 6 and even a part of these are returned forreflux through line 6. The solvent and bottoms hydrocarbons areseparated irom each other in a column 5 by ordinary distillation, thecolumn being equipped with heater II) for this purpose, and theoverhead, consisting of the bottoms hydrocarbons from the original feedto column I, is withdrawn from column 9 through line H, the strippedsolvent 9 being returned to column I through line I. The column I isoperated at temperatures between the boiling points of the solvent andthe hydrocarbons at the existing pressure of the column. At saidtemperatures, the vapor portion consists chiefly of hydrocarbons, andthe liquid portion consists chiefly of solvent, but with appreciablepercentages of hydrocarbons dissolved therein.

The aforesaid operations are conveniently conducted at substantiallyatmospheric pressure, but may be successfully conducted atsubatmospheric pressure or at superatmospheric pressure it preferred,the temperatures depending upon the pressure, the particular solventemployed, the per cent solvent in the liquid reflux, and the compositionof the hydrocarbon mixture in the reflux liquid.

For example, at atmospheric pressure with aniline as the solvent and theoperation maintained with liquid reflux containing mol percent aniline,the temperature in the solvent column then depends only upon thecomposition of hydrocarbons dissolved in the solvent. For these,

conditions, the temperatures for compositions consisting chiefly orentirely of single hydrocarbons in the solvent are approximately:

when two or more such hydrocarbons are pres-'- ent the temperature willbe between the lowest and the highest given for any of them in the abovetabulation.

After a period of use, the solvent becomes contaminated with impuritieswhich may consist of decomposition, oxidation, and/or reaction products,of minor amounts of high-boiling hydrocarbons not removed by the solventrecovery column, of sulfur, nitrogen, oxygen or other bodies present inthe ori inal feedstock, and of water from stripping steam where this isused, and for best results the solvent should be occa- 'sionallywithdrawn from the system and purified by redistillation and/or otherprocesses.

In the practical application of the above described operation tocontinuous recovery of substantially pure hydrocarbons from anarrow-boiling straight-run petroleum fraction stock, using aniline as asolvent, apparatus of the nature of that diagrammatically shown in Fig.2 may be satisfactorily employed and the principal hydrocarbonsrecovered in purities of 99 mol percent. For simplicity, the columns.distillation units and connecting lines are indicated and all auxiliaryapparatus such as reboilers, condensers, heat exchangers, pumps, tanks,valves and control apparatus are omitted.

As an example, I shall now describe the treatment, in accordance withthe invention of an original stock having an A. S. T. M. boiling rangeof 148 F. to 165 F. and ultimate composition as follows:

Vol. percent The narrow-boiling stock is fed to the column of oneextractive distillation unit 2i through line 21. This column separatesthe feed into an overhead containing only parafiins and a bottomscontaining all the cyclics except cyclopentane but with a residue ofhexane. The cyclopentane tends to accumulate in this column and iseliminated by bleeding out a small portion of the vapor at anappropriate point through line 28. The hydrocarbon bottoms product fromthe solvent recovery column passes through line 29 to the column of asecond extractive distillation unit 22 which eliminates all the hexanealong with some methylcyclopentane in its overhead product. Thisoverhead is then recycled to the original feed through line 30 althoughsimilar results are attained by having this overhead enter column 2|directly at a point slightly above line 21 as indicated by the brokenline in Fig. 2. The bottoms product from extractive distillation unit 22passes through line 3| to the column of a third extractive distillationunit 23 which separates pure benzene as bottoms product eliminatedthrough line 32, from the naphthenes. The overhead product, containingonly the two naphthenes, methylcyclopentane and cyclohexane passesthrough line 33 to fractionator 26 which separates it into puremethylcyclopentane overhead, removed through line 34, and purecyclohexane bottoms recovered through line 35.

The paraflln overhead product from column 2| passes to a fractionator 24through a line 36.

The trace of 3-methylpentane tends to accumulate in the column offractionator 24 and is eliminated by bleeding out a small portion of theliquid reflux at an appropriate point through line 31. The bottomsproduct recovered through line 33 is pure hexane.

The overhead from fractionator 24 consists of a mixture of pentane,2,3-dimethylbutane and 2-methylpentane. It passes through a line 39 intothe column of a second fractionator 25, operated to give pure2-methylpentane'as bottoms product recovered through line 40. Theoverhead removed from the fractlonator 25 through a line 4| contains thelow-boiling minor paraiflns 2,3-dimethylbutane and pentane with some2-methylpentane, which it is normally not economical to recover,although obviously they can be recovered by further fractionationprocesses if desired.

As a second illustration, a hexane-hexene mixture from catalyticgasoline whose A. S. T. M. boiling range is 141 F. to 154 F. wasseparated in a laboratory extractive distillation unit into 55% of anoverhead product of substantially pure isomeric hexanes and a 45%bottoms product of substantially pure isomeric hexenes. Said mixturecontained no aromatic hydrocarbons and only small amounts of naphthenesand of unsaturated naphthenes.

Elements of the flow diagrams, e. g., the bleeding out of an impurityand the provision for recirculation of a stream so that a column isrequired to make only one sharp separation have been known and used inthe art of distillation for many years, and I do not claim themindividually as my invention. Furthermore, all the pure hydrocarbonproducts described in the illustrations can be obtained from therespective original mixtures with an apparatus which contains only oneeflicient column,.

if said column be operated intermittently and batchwise with storage ofintermediate products, so that at variou times the column performsessentially the same separations as the six columns indicated in Fig. 2.

It will be recognized from the foregoing that in accordance with myinvention it will generally be preferred to remove from the stock in theoverhead from the first extractive distillation operation all, orsubstantially all, the hydrocarbons which are relatively high inhydrogen content and thus have high activity coefllcients in a solvent,and to leave in the last extractive distillation bottoms for recoverytherefrom in accordance with any suitable method, of which several areknown, the aromatic hydrocarbons which are low in hydrogen content andhave relatively low activity coefficients. Intermediate hydrocarbons,such as naphthenes, unsaturated naphthenes, olefins and diolefins,

which may be considered neither high nor low I in hydrogen content oractivity coefficients may be recovered initially in the overhead fromthe first extractive distillation operation, for further refinement asherein described, or may be retained in the first extractivedistillation bottoms and removed from the solvent in the overhead from asecond or third extractive distillation operation prior to finalrefinement by ordinary fractional distillation for recovery of the puresubstances. The choice in these matters will usually depend upon whatother types of hydrocarbons are present in the original stock and themanner in which it is desired to carry out 'the operation, withparticular reference to the temperatures, pressures and solventsemployed in the recovery. Thus it will be understood that the specificoperations herein described by way of example and to illustrate thepractice of the invention are susceptible of numerous modifications indetail and in the relationships between various contributing factors,many of which are so interrelated that any change in one may require orproduce a corresponding change in others, but these and other changesand modifications will readily occur to those skilled in the art in thelight of the foregoing and may be made without departing from the scopeof the invention as defined in the appended claims. 7

Having thus described my invention, I claim and desire to protect byLetters Patent the United States:

A method for separating hexane, methyl pentane, methyl cyclopentanecyclohexane and benzene individually from a hydrocarbon mixturecontaining them which comprises first extractively distilling themixture in the presence of a substantial concentration of aniline,removing from the first extractive distillation a bottom product and anoverhead product, said overhead product containing hexane and 2-methylpentane, subjecting said overhead product to fractional distillation toseparate therefrom hexane as a bottom product and an overhead productcontaining 2-methyl pentane, subjecting this overhead product tofractional distillation to separate 2-methyl pentane therefrom as abottom product, subjecting the bottom product of said first extractivedistillation to fractional distillation to separate anoverhead productfrom the aniline, returning the aniline to the first extractivedistillation, extractively distilling this last mentioned overheadproduct in the presence of a substantial concentration of aniline toseparate an overhead product con taining hexane and a bottom productcontaining aniline, methyl cyclopentane, cyclohexane and benzene,returning the hexane-containing overhead product to the first extractivedistillation, fractionally distilling said aniline containing bottomproduct to separate an overhead containing methyl-cyclopentane,cyclohexane and benzene from the aniline, returning said aniline to thesecond extractive distillation, extractively distilling saidmethyl-cyclopentaneand-cyclohexane-containing overhead product in thepresence of additional aniline in substantial concentration to separatea third overhead product consisting of methyl cyclopentane andcyclohexane from a bottom product of benzene and aniline, and separatingthe overhead therefrom by fractional distillation into methylcyclopentane as an overhead product and the cyclohexane as a bottomproduct, separating the aniline and benzene-containing product bytractional distillation, and returning the aniline to the thirdextractive distillation.

JOHN GRISWOLD.

\ BEEERENCES crrnn The following references are of record in the file ofthis patent:

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